The importance of the capacitive properties of the systemic vascular bed has been recognized for some time: this property is involved in the regulation of cardiac output, since it affects the filling pressure of the right heart. This proposal aims at investigating the mechanism(s) by which the circulatory system may change capacitive properties and filling pressure to the right heart via the carotid sinus baroreceptor reflex. We will investigate what particular organ or organs significantly contribute to the changes of overall capacitive function of the systemic vascular bed via the baroreceptor reflex by eliminating specific regions one at a time. Those regions which were found to significantly contribute to overall capacitive function will then be studied in detail using two-port analysis techniques to descrive the resistance and capacitance changes by the reflex system. The liver and spleen will be given special attention in an additional series of experiments concerning capacitive property of the systemic vascular bed. In parallel with the above studies we will investigate the resistive and capacitive property of the entire heart-lung compartment. In an additonal set of experiments we will investigate the capacitive and resistive property of the pulmonary vascular bed and their modifications by the barorecptor reflex. In a final set of experiments we will investigate baroreceptor reflex control of capacitive function in unanesthetized chronically instrumented dogs. The chronic dog experiments will be repeated after they are anesthetized and the data compared. Finally, experiments similar to the acute experiments will be performed in these same dogs.